1. Field of the Invention
The present invention relates to improvements in ducting for acoustical speakers and enclosures.
The invention relates generally to loudspeaker enclosures utilized for sound reproduction and particularly to a method and apparatus for more fully utilizing existing driver cone radiated energy for improvement of efficiency and quality of sound.
This invention relates, generally, to speaker cabinets, and more particularly relates to speaker cabinets of the type having more than one speaker positioned therein and being provided with means that allow the sounds emanating from the speakers to mix prior to discharge of the sound by a horn member.
2. Description of the Prior Art
A bass loudspeaker, or woofer, radiates sound both in the forward and rearward directions. One of the purposes of a speaker enclosure is to prevent the cancellation effect of the rear wave of the woofer upon the waves radiated from the front by isolating the forward wave from the rearward wave. Several kinds of enclosures are known in the art:
(a) Infinite Baffle (Air Suspension): An air suspension enclosure is a completely sealed box in which the rear wave is prevented from cancelling the front wave. PA1 (b) Bass Reflex: The bass reflex design utilizes a portion of the rear wave of the woofer to augment the front wave. PA1 (c) Horn Enclosure: In this design, a horn acts as an acoustical transformer that matches the high mechanical impedance of the vibrating diaphragm to the relatively low acoustical impedance of the air at the large mouth of the horn. PA1 (d) Acoustical Labyrinth: This design channels the rear wave from the woofer through a folded passageway so that when the sound finally emerges it is delayed as much as possible and, therefore, reinforces the woofer at the lowest possible frequency.
The use of quality sound systems in both the home and in businesses are often times limited by the size limitations on the speaker enclosures and hence, there has been considerable effort to achieve big sound while utilizing a small enclosure. Various types of ducting has been accomplished in connection with the speaker enclosures in an attempt to effectively extend the frequency response curve at the low end.
Since low frequency response is largely dependent on the loud speaker system resonance, current designs usually rely on an enclosure that is proportionally large in relation to the driver. Stated another way, the larger the enclosure, the lower the frequency resonance. The driver, or any other moving piston in connecting with the enclosure represents an enclosure opening. The smaller the enclosure opening is, again the lower the resonance is. Therefore, reducing the enclosure size means reducing the driver size as well if low frequency performance is to be maintained.
However, in the case of small enclosures, the driver size must be too small to be an efficient radiator if low frequency performance is the objective. Also, power handling ability is decreased with the use of small drivers. Therefore, it is a practice of most small loud speaker system designs to use a larger driver in order to keep efficiency reasonable, trading low frequency performance as a result of the larger effective enclosure openings.
Increasing the mass of a larger speaker in order to obtain lower frequency response has been accomplished by adding a papier mache weight to the center of the speaker cone on a conventional speaker so that speaker may be used in a smaller enclosure. The addition of weight lowers the resonance of the speaker so that when it is coupled to an enclosure the added mass to the loud speaker diaphragm will help to lower the overall resonance of the loud speaker and enclosure together. Although the added weight lowers the resonance of the loud speaker, its ability to reproduce higher frequencies has been traded for the lower resonance.
Often times additional openings will be provided in the enclosure and are connected to ducting within the enclosure in order to tune the overall resonance of the system while allowing the energy from the rear of the loud speaker cone to be added to the front wave which has met with reasonable success.
Another conventional device to further tune the enclosure is by the addition of a passive radiator which serves to transfer sound into the surrounding outside area.
Another problem associated with the use of large speaker assemblies or passive radiators for that matter, is the tendency for these large diaphragms to continue ringing after the electrical signal has been terminated from the driver.
Conventional drivers are mounted in loudspeaker enclosures with the face of the enclosure being utilized as the radiator while the remainder of the enclosure being utilized as the radiator while the remainder of the enclosure is used as a sound or acoustic energy absorption device. In structures of this nature the driver is physically attached to the face plate and the enclosure has walls formed of non-resonant material with a high sound absorption coefficient, the walls of such enclosures being of a relatively high mass and thickness in order to facilitate maximum sound absorption. In addition, these enclosures are usually filled or stuffed with sound absorbent material such as cotton, fiberglass, etc. Such conventional speaker structure intends the radiation of the principal sound from the front of the enclosure and provides for the reduction or control of sounds which emanate from the rear of the driver cone since sounds emanating from the back side of the cone are essentially 190 degrees out of phase with the forward sound and would effectively cancel the forward sound an would effectively cancel the forward sound wave if the two were permitted to co-mingle. This 180 degrees out of phase sound pressure wave is normally referred to as the back wave and, in addition to possessing high orders of audio energy that must be controlled, reacts within the interior of the loudspeaker enclosure (which in reality is a chamber or series of chambers) to create standing waves of high energy sound plus a counterforce of nodes or low energy areas. In addition, any structural material in the vicinity is invaded through the molecular framework of the material by the primary frequencies of the front and back waves plus all of the supporting harmonics thereof, the totality of which creates vibration resonances commensurate with the mass, tension and composition of the material utilized in the enclosure structure.
A profusion of resonances is thus activated by the driver from the driver chamber or chambers, sides, top bottom, back, etc., it being necessary to bring all of these resonances under some semblance of control if the audio reproduction is to be properly presented.
Control of enclosure oriented sound energy has been directly related to the ability to engage and rapidly convert these waves of pressure energy to other forms of energy. The frequency range of audio sound is such that the most practicable means, and hence, the basic control method that has previously emerged, is the conversion of kinetic pressure energy into heat energy. This conversion process involves insertion of materials with very high fiber count into the pathway of the audio wave. In attempting to penetrate the material, the audio wave will cause the individual fibers of the material to vibrate, thus absorbing and converting the audio energy into heat energy. Materials possessing a very high fiber count, such as cotton, fiberglass, particle board and the like are commonly used. Unfortunately, the efficiency of high fiber count material is quite low and no material has yet surfaced which can effectively absorb and dissipate audio frequencies of the size typically used for loudspeakers in sound reproduction systems. Within the state of the art, high degrees of sound absorption can only be realized by developing anechoic conditions. However, the attainment of anechoic conditions requires the use of expensive materials, specialized construction techniques and air volumes of excessively large proportions, all of which tend to make the anechoic application impractical for typical loudspeaker enclosures.
Accordingly, prior practices in the art have only been able to contain the diverse resonances and undesirable sounds within and emanating from loudspeaker enclosures to that level of efficiency and effectiveness constrained by the commonly available high fiber count materials. These materials have of necessity been used regardless of unfavorable mass and weight considerations and even with the recognition that the materials cannot differentiate between desirable and undesirable audio sounds. In spite of the shortcomings attendant to the prior practices thus enumerated, two predominant designs of loudspeaker enclosures have previously emerged and are almost exclusively constitute conventional practice, these designs being describable as the sealed enclosure, better known as the "infinite baffle,"and the ported box enclosure, most commonly referred to as the "bass reflex."
In the infinite baffle design, the backwave is sealed within the enclosure. The concept involves the use of all solid wall, thereby resulting in the rear wave being prevented from engaging the front wave. Further, high fiber construction material is used to stuff the interior of the enclosure, the high fiber count suppressing the many resonances and unwanted enclosure sounds. In practice, the practical size of a sealed enclosure is severely limited in comparison to the length of the sound waves encountered.
Now referring to the prior art, the patent to Pitre, U.S. Pat. NO. 4,031,318, issued Jun. 21, 1977 for "High Fidelity Loudspeaker Systems"shows ducting surrounding the speaker but does not reduce the effective area of the opening.